Microdosing

The definition of microdosing is a dose that is not pharmacologically active and which in any case does not exceed 100 ug. Investigators using microdoses in humans can generally do so with much more limited preclinical tox data than would be acceptable to permit mg ranges of doses in humans. Implicit in this is that the drug is intended for systemic distribution, resulting in extremely low concentrations in tissues, and I wonder how this would work with, for example, a topically applied compound, where the entire 100 ug dose is applied to a very small area? Would limited preclinical, single topical dose, tox data in rodents alone be sufficient to permit human dosing in this way?